Classifications

• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
  • C08J9/00—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof
  • C08J9/16—Making expandable particles
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
  • C08F279/00—Macromolecular compounds obtained by polymerising monomers on to polymers of monomers having two or more carbon-to-carbon double bonds as defined in group C08F36/00
  • C08F279/02—Macromolecular compounds obtained by polymerising monomers on to polymers of monomers having two or more carbon-to-carbon double bonds as defined in group C08F36/00 on to polymers of conjugated dienes
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
  • C08F287/00—Macromolecular compounds obtained by polymerising monomers on to block polymers
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
  • C08J2351/00—Characterised by the use of graft polymers in which the grafted component is obtained by reactions only involving carbon-to-carbon unsaturated bonds; Derivatives of such polymers
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10S525/00—Synthetic resins or natural rubbers -- part of the class 520 series
  • Y10S525/901—Radial block

Definitions

• the present invention relates to an improved expanded foamed bead of a rubber-modified styrene polymer. More particularly, the present invention is concerned with an expanded foamed bead of a rubber- modified styrene polymer, comprising a plurality of closed cells defined by cell walls which constitute a matrix for the bead. Each of the cell walls comprises two surfaces separated by a distance equal to the thickness of the cell wall.
• the matrix comprises: (a) a continuous styrene polymer phase; (b) a rubber phase dispersed in said continuous styrene polymer phase (a), wherein the rubber phase comprises a plurality of substantially flat, butadiene polymer rubber particles each having at least one styrene polymer particle occluded therein; wherein the flat rubber particles are arranged in a specific lamellar configuration along the thickness of the cell wall.
• the expanded foamed bead of a rubber-modified styrene polymer of the present invention is excellent in its ability to retain a foaming agent gas within the closed cells, and can advantageously be employed for producing, for example, packaging materials and cushioning materials or pack ⁇ ings which are improved in resistance to breakage, such as cracking. Discussion Of Related Art
• Expanded foamed beads of a styrene polymer are molded into various shaped products . During the mold ⁇ ing, the expanded foamed beads are further foamed (or expanded) to give an ultimate foamed, molded product.
• the obtained foamed, molded products are widely em ⁇ ployed as, for example, packaging materials and cush ⁇ ioning materials or packings.
• foamed, molded products produced from conventional expanded foamed beads of a styrene polymer are insufficient in resist- ance to breakage, such as cracking.
• Unexamined Japanese Patent Application Laid-Open Specification No. 56-67344 discloses an expanded foamed bead of a styrene polymer having non-oriented rubber particles dispersed therein.
• Japanese Patent Application Laid-Open Specification No. 56-67344 discloses an expanded foamed bead of a styrene polymer having non-oriented rubber particles dispersed therein.
• Patent document a foamed, molded product produced from the expanded foamed beads of a rubber-modified styrene polymer disclosed therein is described to have an improved impact strength.
• the rubber particles dispersed in the styrene polymer are of a non-oriented type, it is difficult for the rubber particles to change in conformity with the formation of cells during the expansion of the rubber-modified styrene polymer. Therefore, during the expansion of the rubber-modified styrene polymer, the rubber parti- cles are likely to be disadvantageously exposed on the surfaces of cell walls or protrude into the cells, thereby adversely influencing the cells.
• Unexamined Japanese Patent Application Laid-Open Specification No. 63-175043 discloses an expanded foamed bead which is uniform in cell size, and this foamed bead is made from a styrene polymer obtained by polymerizing a solution of a ⁇ tyrene-butadiene block copolymer in a styrene monomer.
• Unexamined Japanese Patent Application Laid-Open Specification No. 2-311542 discloses an expanded foamed bead improved in the strength of a molded product, in which the bead is made from a styrene polymer obtained by polymerizing a solution of a styrene-soluble rubber in styrene.
• foamed, molded products produced from the expanded foamed beads disclosed in these two Japanese Patent documents are unsatisfactory in resistance to cracking.
• the cracking resistance of a foamed, molded product depends on various physical properties of the foamed, molded product, such as compressive strength, tensile strength and elongation, and the structure of cells in the foamed, molded product. These various physical properties of and the structure of cells in the foamed, molded product are influenced by the dis ⁇ persion morphology of the rubber particles dispersed in the styrene polymer constituting the cell walls in expanded foamed beads used for producing the foamed, molded product.
• Specification No. 3-182529 discloses an expanded foamed bead of a resin prepared by mechanically blending a high impact polystyrene and a hydrogenated styrene- butadiene block copolymer.
• a rubber component is mechanically blended with a resin, the dispersion of the rubber component in the resin is likely to be uneven, so that the dispersion of the rubber component in the cell walls of an expanded foamed bead of the resin inevitably becomes uneven.
• the uneven dispersion of the rubber component in the cell walls is likely to cause breakage of the cell walls due to the presence of large aggregated rubber particles and form a large number of open cells, which allow a foaming agent gas to immediately escape there- from.
• the breakage of the cell walls is especially prevalent in the case of a highly expanded foamed bead in which the thickness of the cell walls is small, so that the expanding capability of the expanded foamed bead is lowered.
• an expanded foamed bead having a low expanding capability is subjected to molding under foaming, the resultant foamed, molded product inevita ⁇ bly has undesired voids between those beads, which are formed during the molding due to the unsatisfactory expansion of the expanded foamed beads.
• Such a molded product having voids has a poor appearance.
• the expanded foamed bead has a high ratio of closed cells and is excellent in the retention of a foaming agent gas. Further, it has surprisingly been found that when this expanded foamed bead is used for produc- ing an ultimate foamed, molded product, the resultant molded product is not only excellent in resistance to cracking, but also has an excellent appearance. Thus, the foamed, molded product can advantageously be em ⁇ ployed as, for example, packaging materials and cush- ioning materials or packings. Based on these novel findings, the present invention has been completed.
• an object of the present invention to provide an expanded foamed bead of a rubber-modified styrene polymer, which has a high ratio of closed cells even at a high degree of expansion and is excellent in the retention of a foaming agent gas and which can advantageously be employed for producing a foamed, molded product exhibiting not only excellent resistance to cracking but also an excellent appear- ance.
• FIG. 1 shows an enlarged diagrammatic view of cross-sections of the cell wall of the expanded foamed bead of the present invention, taken along two planes which extend in the direction of the thickness of the cell wall and are transverse to each other, illustrat- ing the dispersion morphology of substantially flat rubber particles dispersed in a continuous styrene polymer phase in the cell wall in the foamed bead of the present invention;
• Fig. 2(a) shows an enlarged diagrammatic view of the foamed bead of the present invention with a portion thereof cut away to show a cross-section of the foamed bead;
• Fig. 2(b) shows an enlarged diagrammatic view of a portion of the cross-section of Fig. 2(a), which is encircled by a broken line in Fig. 2(a);
• Fig. 3(a) shows a diagrammatic view illustrating the manner of protecting an article with a set of four packings (ultimate foamed, molded products) produced from the expanded foamed beads of the present invention for packaging the article in a container (the article protected and packaged in the container is to be sub ⁇ jected to drop testing);
• Fig. 3(b) shows a diagrammatic view of the con ⁇ tainer having the article protected and packaged there- in in the manner shown in Fig. 3(a), showing three edges and one corner which will be in collision with a floor in the drop testing).
• an expanded foamed bead of a rubber-modified styrene polymer comprising a plurality of closed cells defined by cell walls which constitute a matrix for the bead, each of the cell walls comprising two surfaces separated by a distance equal to the thickness of the cell wall, the matrix comprising:
• Fig. 1 shows an enlarged diagrammatic view of 20 cross-sections of the cell wall of the expanded foamed bead of the present invention, taken along two planes which extend in the " direction of the thickness of the cell wall and are transverse to each other.
• the figure illustrates the dispersion morphology of substantially 25 flat rubber particles dispersed in a continuous styrene polymer phase in the cell wall in the foamed bead of the present invention.
• numeral 1 designates a cell wall
• numeral 2 designates a surface of cell wall 1
• numeral 3 designates continuous styrene polymer phase (a)
• numeral 4 designates rubber particle (b)
• numeral 5 designates a styrene polymer particle occlud ⁇ ed in rubber particle (b)
• characters a and b designate the lengths of the short and long axes of the cross- section of rubber particle (b), respectively
• character c designates the thickness of the cross- section of cell wall 1.
• Fig. 2(a) shows an enlarged diagrammatic view of the foamed bead of a rubber-modified styrene polymer 6 of the present invention with a portion thereof cut away to show a cross-section of the foamed bead.
• numeral 6 designates the foamed bead of the present invention and numeral 7 designates a closed cell in the foamed bead.
• Fig. 2(b) shows an enlarged diagrammatic view of a portion of the cross-section of Fig. 2(a), which is encircled by a broken line in Fig. 2(a).
• numeral 7 designates a closed cell in the foamed bead and numeral 1 designates a cell wall.
• the expanded foamed bead 6 of the present inven- tion comprises a plurality of closed cells 7 defined by cell walls 1 which constitute a matrix for the bead 6, each of the cell walls 1 comprising two surfaces 2 (only one surface is shown in Fig. 1) separated by a distance equal to the thickness c of the cell wall 1.
• the matrix comprises:
• a rubber-modified styrene polymer when jolted or impacted, it exhibits a resistance to cracking.
• the cracking resistance of a rubber-modified styrene polymer is due to the presence of a rubber phase dispersed in a styrene polymer phase, the rubber phase serving to suppress the spread of a cracking caused by the impact on the styrene polymer phase.
• the configuration of the rubber particles, the molecular orientation of the rubber particles, and the molecular orientation of the styrene polymer are completely different from those in the non-foamed, rubber-modified styrene polymer. Therefore, between a non-foamed, rubber-modified sty- rene polymer and a molded product produced from a foamed bead of the rubber-modified styrene polymer, there is a large difference in the type of dispersion morphology of the rubber phase suitable for improving a cracking resistance.
• the present inventors have for the first time found that when the rubber particles dis ⁇ persed in the continuous styrene polymer phase are arranged in a specific lamellar configuration along the thickness of the cell wall and the dimensions of each rubber particle and the thickness of the cell wall satisfy specific relationships, the foamed bead has a high ratio of closed cells even at a high degree of expansion or foaming and is excellent in the retention of a foaming agent gas, and a molded product produced therefrom is not only excellent in resistance to crack ⁇ ing, but also has an excellent appearance.
• the flat rubber particles 4 are arranged in lamellar con- figuration along the thickness of the cell wall 1 and are oriented so that the long axis of a cross-section of each flat rubber particle 4, as viewed in a cross- section of the cell wall 1, taken along the thickness of the cell wall 1, is substantially parallel to the two surfaces 2 of the cell wall 1, and the cell wall 1 and each flat rubber particle 4 satisfy the formulae I and II:
• a represents the thickness ( ⁇ m) of the rubber particle as measured in terms of the length of the short axis of the cross-section of the rubber particle
• b represents the diameter ( ⁇ m) of the rubber particle as measured in terms of the length of the long axis of the cross-section of the rubber particle
• c represents the thick ⁇ ness ( ⁇ m) of the cross-section of the cell wall.
• the aspect ratio b/a i.e., the ratio of the diameter ( ⁇ m) of the rubber particle, as measured in terms of the length of the long axis of the cross-section of the rubber particle, to the thickness ( ⁇ m) of the rubber particle as meas ⁇ ured in terms of the length of the short axis of the cross-section of the rubber particle
• the aspect ratio b/a which represents the flatness of a cross-section of a flat rubber parti ⁇ cle, as viewed in a cross-section of the cell wall, taken along the thickness of the cell wall
• the ratio b/a is in the range from 10 to 70.
• the ratio b/a is preferably in the range from 10 to 40.
• the ratio b/a is less than 10
• the rubber particles tend to be exposed on the surfaces of the cell walls, so that the retention of a foaming agent gas in the cells is lowered.
• the ratio b/a is larger than 70, the thickness of the rubber particle is excessively small relative to the size of the surface of the rubber particle, resulting in a lowering of the ability of the rubber particles to suppress the spread of a crack through ' the styrene polymer phase, so that the cracking resistance of an ultimate foamed, molded product is lowered.
• the ratio a/c i.e., the ratio of the thickness ( ⁇ ) of the rubber particle, as measured in terms of the length of the short axis of the cross-section of the rubber particle, to the thick ⁇ ness ( ⁇ m) of the cross-section of the cell wall] is a value obtained as an average of the values of ratio a/c between the cross-section of the cell wall and each of the 20 rubber particles randomly selected in the cross-section of the cell wall.
• the ratio a/c is in the range from 0.01 to 0.2.
• the ratio a/c is preferably in the range from 0.01 to 0.1.
• the ratio a/c is less than 0.01, the thickness of the rubber particle is too small relative to the thickness of the cell wall, resulting in a lowering of the abili- ty of the rubber particles to suppress the spread of a crack through the styrene polymer phase, so that the cracking resistance of an ultimate foamed, molded product is lowered.
• the ratio a/c is larger than 0.2, the rubber particles tend to be exposed on the surfaces of the cell walls, so that the retention of a foaming agent gas in the cells is low ⁇ ered.
• the thickness of the cross-section of the cell wall is preferably in the range from 0.2 to 10 ⁇ m, more preferably from 0.3 to 5 ⁇ m.
• the number of the flat, butadiene polymer rubber particles which are arranged in lamellar configuration along the thickness of the cell wall is preferably from 2 to 20, more preferably from 2 to 10.
• the flat particles are oriented so that the long axis of a cross-section of each flat rubber particle, as viewed in a cross-section of the cell wall, taken along the thickness of the cell wall, i ⁇ substantially parallel to the two surfaces of the cell wall.
• the values a, b and c can be measured by the following method:
• a portion of an expanded foamed bead is cut away to expose a cross-section thereof; the foamed bead having a cross-section exposed is immersed in an aque ⁇ ous 2 % osmium tetrachloride solution for 24 hours to thereby stain the cross-section, followed by a washing with distilled water; the foamed bead is embedded in an epoxy resin which can be cured at room temperature; an ultrathin slice is cut out from the cross-section of the embedded foamed bead, using an ultramicrotome; and an electron photomicrograph of the ultrathin slice is taken; and the values a, b and c are measured on the electron photomicrograph.
• the configuration of a flat rubber particle as viewed in a direction perpendicular to the surfaces of the cell wall is not specifically limited and can be varied, for example, a circular, elliptic or polygonal configuration.
• foamed bead of the present invention There is no particular limitation with respect to the configuration of the foamed bead of the present invention.
• morphologies of the foamed bead include a sphere, a cylinder, and an ellipsoid.
• the foamed bead have an apparent density of from 0.014 to 0.100 g/cm . It is more preferred that the apparent density be from 0.014 to 0.07 g/cm 3 .
• the apparent density is less than 0.014 g/cm , the ratio of closed cells of the foamed bead is low, and also the strength of an ultimate foamed, molded product is low.
• the apparent density is larger than 0.100 g/cm , the amount of the rubber-modified styrene polymer used is disadvantageously increased, so that the cost is increased.
• the fact that the flat rubber particles are arranged in specific lamellar configuration along the thickness of the cell wall means that in the course of the foaming of the rubber-modified styrene polymer toward the formation of the foamed bead, i.e., in the course of the development and expansion of cells, which causes the cell walls to be stretched, the dispersed rubber particles are appropriately oriented in conformity with the stretching of the continuous styrene polymer phase.
• the specific lamellar configuration of the arrangement of the flat rubber particles can be attained by appro- priately selecting a relationship between the viscoe- lasticity of the continuous styrene polymer phase and that of the rubber phase.
• the viscoelasticity of a rubber is varied depending on the cross-linking degree, the molecular weight and the like.
• the viscoelasticity 5 of a styrene polymer is varied depending on the molecu ⁇ lar weight and the like. From the viewpoint of attain ⁇ ing an appropriate relationship between the viscoelas ⁇ ticity of the continuous styrene polymer phase and that of the rubber phase, in the foamed bead of the present o invention, it is preferred that the continuous styrene polymer phase (a) have an intrinsic viscosity of from 0.6 to 0.9 dl/g as measured in toluene at 30 °C, and the matrix have a gel moiety with a swelling index of from 6.5 to 13.5, the gel moiety being defined as an 5 extraction residue of the extraction of the foamed bead with toluene at 25 °C, the swelling index of the gel moiety being defined as a value (B) obtained according to the formula III:
• the intrinsic viscosity of the continuous styrene polymer phase is less than 0.6, the molecular weight of the continuous styrene polymer phase is too low, so that the flowability of the continuous styrene polymer phase becomes large and the strength of the continuous styrene polymer phase is lowered.
• the Q intrinsic viscosity of the continuous styrene polymer phase is larger than 0.9, it is difficult to prepare an appropriate rubber-modified styrene polymer, using such a styrene polymer. It is more preferred that the intrinsic viscosity of the continuous styrene polymer phase be from 0.65 to 0.85.
• the swelling index of the gel moiety When the swelling index of the gel moiety is less than 6.5, the cross-linking degree of the gel moiety is too high, so that the thickness of each rubber particle is less likely to become appropriately small in thick ⁇ ness in the course of the formation of a foamed bead.
• the swelling index of the gel moiety is larger than 13.5, the cross-linking degree of the gel moiety is too low, the elongation becomes insufficient, so that the cracking resistance is poor. It is more preferred that the swelling index of the gel moiety be from 8.5 to 12.5.
• the cross-linking of the rubber phase occurs during the production of the rub ⁇ ber-modified styrene polymer.
• the resultant reaction mixture is intro ⁇ quinted to a volatilization apparatus which is heated (at 150 °C or more) in vacuo, to thereby remove the unre- acted styrene, thus obtaining a rubber-modified styrene polymer.
• the rubber phase undergoes cross-linking by heat.
• the flat rubber particle be comprised of at least one butadiene polymer selected from the group consisting of a polybutadiene and a styrene-butadiene block copolymer.
• the continuous styrene polymer phase (a) be comprised of at least one styrene polymer selected from the group consisting of a po ⁇ lystyrene and a styrene copolymer having a styrene content of 50 % by weight or more.
• the rubber-modified styrene polymer used in the present invention is a composition in which butadiene
• polymer rubber particles are dispersed in a styrene polymer.
• dispersion of butadiene polymer rubber particles in a styrene polymer can be attained by either (1) a method in which a butadiene polymer rubber is dissolved in a styrene monomer or a mixture
• the dispersing method (1) above non-mechanical dispersion
• rubber particles can be evenly dis ⁇ persed.
• each rubber particle has either (1) a core-shell structure wherein a single particle of a styrene polymer is occluded as a core in the rubber particle which constitutes a shell, or (2) a structure wherein at least two styrene polymer particles are occluded in the rubber particle (the so- called "salami" structure).
• the rubber phase is comprised of rubber particles having a core-shell structure, a salami structure, or a combina ⁇ tion thereof.
• the rubber phase be comprised of rubber particles each having a diameter of 1 ⁇ m or less and having a core-shell structure.
• each (core-shell structure) rubber particle have a diameter of from 0.1 to 1 ⁇ m, more preferably from 0.1 to 0.5 ⁇ m.
• the dispersed rubber phase be comprised of a mixture of 80 % by weight or more, based on the weight of the rubber phase, of rubber particles each having a diameter of 1 ⁇ or less and having a core-shell structure wherein a single particle of a styrene polymer is occluded as a core in the rubber particle which constitutes a shell, and 20 % by weight or less, based on the weight of the dispersed rubber phase, of rubber particles each having a salami struc- ture wherein at least two styrene polymer particles are occluded in each rubber particle.
• each rubber particle having a core-shell structure have a diameter of from 0.1 to 1 ⁇ m, more preferably from 0.1 to 0.5 ⁇ m.
• the rubber-modified styrene poly ⁇ mer to be used in the present invention when a rubber particle has a core-shell structure and has a diameter of 1 ⁇ m or less, even dispersion of flat rubber parti ⁇ cles in the cell walls of a foamed bead can be ob- tained.
• rubber particles each having a diameter as small as 1 ⁇ m or less are suitable for facilitating even dispersion of flat rubber particles in the cell walls of a foamed bead.
• rubber particles each having a salami structure wherein at least two styrene polymer parti ⁇ cles are occluded in each rubber particle the particle diameter thereof tends to be more than 1 ⁇ , so that it is somewhat difficult to obtain an even dispersion of flat rubber particles in the cell walls of a foamed bead.
• rubber parti ⁇ cles each having a salami structure may be used alone or in combination with rubber particles each having a core-shell structure.
• each rubber particle, in the non-foamed rubber-modified styrene polymer to be used in the present invention is not particularly limited.
• Examples of configurations of each rubber particle in the non-foamed rubber-modified styrene polymer include a sphere, an ellipsoid, and an irregular shape.
• each flat rubber parti ⁇ cle have a core-shell structure wherein a single parti- ole of a styrene polymer is occluded as a core in the rubber particle which constitutes a shell.
• the dispersed rubber phase be comprised of a mixture of 80 % by weight or more, based on the weight of the dispersed rubber phase, of flat rubber particles each having a core-shell structure wherein a single particle of a styrene polymer i ⁇ occluded as a core in the rubber particle which consti ⁇ tutes a shell, and 20 % by weight or less, based on the weight of the dispersed rubber phase, of flat rubber particles each having a salami structure wherein at least two styrene polymer particles are occluded in each rubber particle.
• the average weight of the foamed bead of the present invention there is no particular limitation with respect to the weight of the foamed bead of the present invention. However, it is preferred that the average weight of the foamed bead be from 0.2 to 2 mg, more preferably from 0.4 to 1.2 mg.
• the term "average weight of the foamed bead" means a value obtained as an average weight of 200 foamed beads randomly selected.
• a proce ⁇ s for producing an expanded foamed bead of a rubber-modified styrene polymer of the present invention which comprises: o (1) melt-kneading a rubber-modified styrene polymer with a foaming agent in an extruder to form a molten mixture therein, the rubber-modified styrene polymer comprising:
• a continuous styrene polymer phase (a) a continuous styrene polymer phase; 5 (b) a rubber phase dispersed in the continuous styrene polymer phase (a), comprising a plurality of butadiene polymer rubber particles each having at least one styrene polymer particle occluded therein, wherein the continuous styrene polymer phase (a) has an intrin- sic viscosity of from 0.6 to 0.9 dl/g, and the rubber- modified styrene polymer ha ⁇ a gel moiety with a swell- ing index of from 6.5 to 13.5, the gel moiety being defined as an extraction residue of the extraction of the rubber-modified styrene polymer with toluene at 25 °C, the swelling index of the gel moiety being defined as a value (B) obtained according to the formu ⁇ la III:
• W represents the weight of the gel moiety swelled with toluene at 25 °C
• W 2 represents the weight of the gel moiety obtained by drying the swelled gel moiety
• the rubber-modified styrene polymer to be used for producing an expanded foamed bead of the present inven ⁇ tion can be prepared by a customary method, such as a bulk polymerization method, a combined method of bulk polymerization and suspension polymerization, or an irradiation polymerization method.
• the bulk polymerization method can be practiced as follows:
• a butadiene polymer rubber is dissolved in a styrene monomer and the resultant solution is sub ⁇ jected to polymerization at an elevated temperature while stirring.
• butadiene polymer rubbers examples include polybutadienes (including low-cis polybutadiene having a cis-1,4 addition content of 35 I, a trans-1,4 addi ⁇ tion content of 52 % and a 1,2 addition content of 13 %; and high-cis polybutadiene having a cis-1,4 addition content of 90 to 98 i, a trans-1,4 addition content of 1 to 4 % and a 1,2 addition content of 1 to 6 %), a styrene-butadiene copolymer (a random SBR and a block SBR), a polyi ⁇ oprene, and a butadiene-i ⁇ oprene block copolymer. Of these examples, polybutadiene and a styrene-butadiene block copolymer are preferred. These butadiene polymer rubbers can be employed alone or in combination.
• styrene monomers include styrene; styrene derivatives in which an alkyl group is attached as a substituent to the benzene nucleus, such as o- methylstyrene, p-methylstyrene, m-methylstyrene, 2,4- dimethylstyrene and ethylmethylstyrene; -alkyl- substituted styrene derivative ⁇ , such as ⁇ - methylstyrene; and styrene derivatives in which a halogen atom is attached as a substituent to the ben ⁇ zene nucleus, such as o-chlorostyrene. These styrene monomers can be used alone or in combination.
• At least one monomer other than a styrene monomer may be employed as a comonomer.
• comonomers include acrylonitrile, methyl methacrylate, and maleic anhydride.
• solvents include aromatic hydrocarbons, such as toluene, xylene, and ethylbenzene. These solvents can be used alone or in combination.
• the polymerization reaction can be effected simply by heating at a temperature of from 100 to 180 °C without using a polymerization initiator.
• a polymeri ⁇ zation initiator may preferably be employed.
• initiators include peroxyketals, such as l,l-bis(t- butylperoxy)cyclohexane; dialkyl peroxides, such as di-t-butyl peroxide; diaryl peroxides, such as benzoyl peroxide; peroxydicarbonates; peroxyesters; ketone peroxides; and hydro peroxides.
• a chain transfer agent may be used for the poly ⁇ merization reaction.
• chain transfer agents include -methylstyrene dimer; mercaptans, such as n- dodecyl mercaptan, t-dodecyl mercaptan, 1-phenylbutene- 2-fluorene, dipentene and chloroform; terpenes; and halides.
• the reaction temperature is generally in the range from 50 to 170 °C, preferably from 90 to 155 °C.
• the reaction temperature may be constant or may be gradual ⁇ ly elevated during the reaction.
• the temperature may, for example, be elevated stepwise in a manner such that the temperature is elevated 2 or more times at a temperature elevation rate of 0.2 to 2 °C/minute, preferably 0.4 to 1.5 °C/minute.
• the reaction is continued until a desired conversion has been achieved.
• the unreacted monomer and any solvent used are removed by, for example, heating in vacuo to obtain a rubber- modified styrene polymer.
• the obtained rubber-modified styrene polymer is continuously supplied to an extruder.
• the supplied polymer is heat-melted and extruded into strands through orifices provided at a die of the extruder.
• the extruded strands are immediately cooled in a water bath, while being received between upper and lower drive rolls, which send the cooled polymer strands to a rotary cutter.
• the rotary cutter cuts the polymer strands in a transverse direction at predetermined intervals, thereby obtaining polymer beads.
• a styrene-butadiene block copolymer As a rubber compo ⁇ nent, first, a styrene-butadiene block copolymer is dissolved in a styrene monomer and then, the styrene monomer is polymerized (see Example 1).
• the styrene-butadiene block copolymer When the styrene-butadiene block copolymer is dissolved in the styrene monomer, the styrene polymer blocks of the block copolymer bind together, whereas the butadiene polymer blocks bind together. Since the continuous phase being formed by polymerization is of a styrene polymer, the styrene polymer blocks bound together are unified into the continuous.polystyrene phase, while a portion of the styrene monomer enters into the butadi ⁇ ene polymer blocks bound together, so that the butadi ⁇ ene polymer blocks together form a shell around a core formed of a single particle of the styrene polymer. In general, for efficiently forming dispersed rubber particles having a core-shell structure, there are examples of the styrene polymer blocks bound together.
• 0 can be used, for example, a method in which the affini ⁇ ty of the butadiene polymer to the styrene polymer is enhanced; a method in which the viscosity of a feed stock solution to be used for polymerization is appro ⁇ priately adjusted; a method in which the rate and time ,-. of stirring the reaction system during polymerization are appropriately adjusted; and a method in which uniform stirring of the reaction system is conducted. That is, formation of dispersed rubber particles having a core-shell structure can be achieved by appropriately
• a polybutadi ⁇ ene as a rubber component. That is, for preparing a feed stock solution for polymerization, first, a poly ⁇ butadiene is dissolved in a styrene monomer and then, the styrene monomer is polymerized (see Example 2).
• the o polybutadiene forms particles while a portion of the styrene monomer enters into polybutadiene particles being formed, so that a plurality of styrene polymer particles are occluded in each butadiene rubber parti ⁇ cle.
• a combined method of bulk polymerization and suspension polymerization can also be utilized. In this method, a bulk polymerization is conducted in the early stage, and a suspension polymer- o ization is conducted in the later stage.
• a butadiene polymer is dissolved in a styrene monomer to obtain a solution for polymerization.
• 10 to 40 % by weight of the styrene monomer are polymerized with the butadiene polymer in 5 the same manner as in the above-mentioned bulk polymer- ization, to thereby obtain a mixture of a partially polymerized styrene polymer and the remaining styrene monomer.
• the obtained mixture is stirred and dispersed in an aqueous medium in the presence of a suspension stabilizer and a surfactant, and subsequently a suspen ⁇ sion polymerization is allowed to proceed in the later stage.
• the resultant rubber-modified styrene polymer is washed and dried. If desired, the obtained polymer can be formed into pellets or a powder.
• Additives can be added to the rubber-modified styrene polymer, such as a dye, a pigment, a lubricant, a filler, a releasing agent, a plasticizer, an anti ⁇ static agent, a foam-nucleating agent and a stabilizer to ultraviolet rays, as well known in the art.
• an expandable, foamable polymer particle, an expanded foamed bead of a rubber- modified styrene polymer of the present invention and an ultimate foamed, molded product can be obtained as follows:
• the above-obtained rubber-modified styrene polymer is impregnated with a foaming agent.
• a foaming agent for impregnating a rubber-modified styrene polymer with a foaming agent and molding the resultant impregnated polymer with .in situ foaming and expansion, there can be advantageously used a so-called extrusion/impregnation method.
• the rubber- modified styrene polymer is heat-melted in an extruder.
• a volatile foaming agent is introduced under pressure through a feeding line which is connected to the extruder.
• the rubber-modified styrene polymer is melt-kneaded with the foaming agent in the extruder.
• the resultant molten mixture is retained in the extruder for 15 minutes or more, preferably 20 minutes or more and, subsequently, the molten mixture is extruded into strands through orifices provided at a die of the extruder.
• the extruded strands are immediately cooled in a water bath, while being received between upper and lower drive rolls, which send the cooled polymer strand ⁇ to a rotary cutter.
• the rotary cutter cut ⁇ the polymer strands in a transverse direction at predetermined intervals, thereby obtaining polymer beads.
• the molten mixture can be extruded into water, and the extrudate can be cut in the water immediately upon extrusion. This method is preferred because the pieces obtained by cutting can easily become spherical.
• the retention time of the molten mixture of a rubber-modified styrene polymer and a foaming agent under the above-mentioned specific conditions can be regulated by providing a conduit between an extruder and a die thereof and appropriately adjusting the length of the conduit.
• the retention time can be regulated by appropriately adjusting the rate of the extrusion.
• foaming agent-impregnated, rubber-modified styrene polymer beads to be used for producing foamed beads of the present invention can also be obtained by a so-called suspension/impregnation method.
• beads of the rubber-modified styrene polymer are dispersed in an aqueous medium in the presence of a suspension stabilizer and a surfact ⁇ ant while agitating.
• a foaming agent is introduced to the aqueous medium under a pressure of, for example, about 50 kg/cm 2 G, while heating at room temperature to about 120 °C, to thereby impregnate the rubber-modi ⁇ fied styrene polymer with the foaming agent.
• volatile foaming agents to be used for producing expandable, foamable pellets or particles of the present invention, include aliphatic hydrocarbons, such as propane, butane, pentane, hexane, heptane and petroleum ether; alicyclic hydrocarbons, such as cyclo- pentane and dichlorohexane; and halogenated hydrocar- bons, such as methyl chloride, ethyl chloride, methyl bromide, dichlorodifluoromethane, 1,2-dichlorotetra- fluoroethane and onochlorotrifluoroethane.
• aliphatic hydrocarbons such as propane, butane, pentane, hexane, heptane and petroleum ether
• alicyclic hydrocarbons such as cyclo- pentane and dichlorohexane
• halogenated hydrocar- bons such as methyl chloride, ethyl chloride, methyl bromid
• the foamable pellets or particles of a rubber- modified styrene polymer impregnated with a foaming agent can be expanded and foamed using steam by means of a conventional machine used for producing foamed polystyrene beads to obtain expanded foamed beads of a rubber-modified styrene polymer.
• heating is conducted at a temperature of from 95 to 104 °C for 10 to 150 seconds, preferably for 20 to 60 seconds.
• the expanded foamed beads of a rubber modified styrene polymer of the present invention can be sub ⁇ jected to molding by means of a conventional molding machine for producing foamed, molded polystyrene products, in which the expanded foamed beads are fu- sion-unified with in situ expansion and foaming, so that an ultimate foamed, molded product can be ob ⁇ tained.
• the apparent density of an expanded foamed bead is obtained as follows: o About 5 g of expanded foamed beads of a rubber- modified styrene polymer are weighed with an accuracy on the order of 0.01 g. Separately, from 50 to 100 cm J of water is poured into a 200 cm J glass measuring cylinder having a minimum graduation unit of 1 c ⁇ . A 5 pushing tool is submerged in the water, which is con- structed of a circular wire net having a diameter slightly smaller than the inner diameter of the cylin ⁇ der and a wire (having a length of from 15 to 30 cm) vertically extended from the center of the net. Then, the water level is read to obtain level H (cm J ) .
• a bulk density of a foamed, molded product, D (g/cm 3 ), is calculated by the following formula in accordance with JIS K6767:
• a portion of a foamed, molded product is cut out to obtain a rectangular parallelopiped as a sample.
• the weight [G(g)] of the rectangular parallelopiped (weight of the sample) is measured, and then the length, width and height of the rectangular parallelo ⁇ piped are measured (in cm), by means of a measuring tool as described in JIS K6767 with an accuracy as also described in JIS K6767.
• the volume of the foamed, molded product sample is obtained by calculating a product of the length multiplied by the width and height.
• V ⁇ represents a sum of the volume of the matrix of the foamed bead and the total inner volume of the closed cells in the foamed bead
• V a represents a sum of the volume of the matrix of the foamed bead
• W represents the weight of the foamed bead
• p represents the density of the matrix
• n ⁇ represents the number of rubber particles having a diameter D ⁇
• D_ represents the diameter (with an accuracy on the order of 0.1 ⁇ m)
• diameter D ⁇ is defined as (L + L 2 )/2 wherein I-2 is a major diameter of the particle, and L 2 is a minor diameter of the particle.
• minor diameter means the maximum of distances between opposite points on the periphery of the non- circular shape, wherein the opposite points are posi- tioned on opposite sides of a longitudinal axis of the non-circular shape (which is defined as a central line extending along the length of the non-circular shape), and the terminology “major diameter” used herein means the maximum of distances between opposite points on the periphery of the non-circular shape, wherein the oppo ⁇ site points are positioned on opposite sides of a short axis of the non-circular shape (which is defined as a central line extending in a direction perpendicular to the above-mentioned longitudinal axis).
• 0.1 g of the thus obtained precipitate is dis ⁇ solved in toluene to thereby obtain a 0.5 g/dl solu ⁇ tion.
• 10 ml of the thus obtained solution is applied to a Cannon-Fenske viscometer (manufactured and sold by Canon, Inc., Japan), and a period of time [t2(sec)] necessary for all of the solution to downwardly flow out is measured at 30 °C.
• a period of time [t Q (sec)] necessary for 10 ml pure toluene to downwardly flow out is meas ⁇ ured at 30 °C.
• the reduced viscosity, ⁇ sp /C of the 0.5 g/dl is calculated according to the following formula:
• ⁇ sp C ⁇ 1 " t 0 )/(t 0 .C) wherein C represents the styrene polymer concentration (g/dl) of the solution.
• 0.5 g of foamed beads is immersed in 30 ml of toluene at 25 °C for 24 hours and then shaken for 5 hours to obtain a mixture.
• the obtained mixture is subjected to centrifugation at 5 °C and 18,000 rpm for 1 hour to thereby separate the mixture into a superna ⁇ tant and a precipitate.
• the supernatant is removed by decantation to obtain a residue.
• Thirty ml of toluene is added to the obtained residue and then shaken at 25 °C for 1 hour.
• the resultant mixture is subjected to centrifugation at 5 °C and 18,000 rpm for 1 hour to thereby separate the mixture into a supernatant and a precipitate.
• the supernatant is removed by decantation to obtain a residue.
• the obtained residue is weighed to obtain a weight (W- j .
• the residue is vacuum-dried.
• the weight (W 2 ) of the dried residue is measured.
• the swelling index, B(%) is calculated according to the following formula:
• Foamable beads are foamed with expansion to obtain expanded foamed beads each having an apparent density of 0.033 g/cm .
• the obtained foamed beads are allowed to stand in a drying container at 23 °C for about 24 hours to thereby remove the moisture present in the surface and inside of the foamed beads.
• the thus dried foamed beads are subjected to measurement of the reten ⁇ tion of the foaming agent in the foamed beads.
• the retention of a foaming agent in an expanded, foamed bead of the present invention can be measured as follows:
• G 100 • (G ⁇ - G 2 )/(G 1 - G 0 ) wherein G Q represents the weight of the glas ⁇ fla ⁇ k; G- ⁇ represent the weight of the glass flask having the beads placed therein, before vacuum-deaeration; and G 2 represents the weight of the glass flask having the beads placed therein, after vacuum-deaeration.
• the content (g/100 g of expanded foamed beads) of the foaming agent in the expanded foamed beads is measured every 3 hours.
• This half-period is taken as the ability of expanded foamed beads to retain the foaming gas (retention of foaming gas).
• voids formed between the foamed beads fused to constitute the foamed, molded product
• the number of those voids which have a size equal to or larger than a half of the size of the foamed bead is counted.
• Fig. 3(a) article 12 is protected with a set of four pack ⁇ ings 8, 9, 10 and 11 which are made of foamed, molded products.
• the packings made of these foamed, molded products are used for protecting articles having weights of 30, 10, 20 and 35 kg, respectively.
• the packings are designed so that the article receives static stresses of 0.08 to 0.12 kg/cm , respectively, from six faces, i.e., front and back faces, left and right faces and upper and lower faces of the packings.
• the article protected by the four packings is accommodated in container 13 [shown in Fig. 3(b)] made of a corrugated board.
• the container having the arti ⁇ cle accommodated therein is dropped with corner 14 downward.
• packing 8 undergoes the great ⁇ est load.
• the container is dropped one time with each of three edges 15, 16 and 17 downward, that is, dropped three times in total.
• the con ⁇ tainer is dropped one time with each of six faces of the container downward, that is, dropped six times in total.
• the container is opened.
• the four pack ⁇ ings are examined with respect to the degree of damage.
• a styrene-butadiene block copolymer having a butadiene content of 60 wt% was dissolved in monomeric styrene so that the concentration of the styrene- butadiene block copolymer in the resultant solution became 12 wt%.
• To 100 parts by weight of the obtained solution were added 5 parts by weight of ethylbenzene, 0.05 part by weight of 1,1-bi ⁇ (t-butylperoxy)cyclohex- ane and 0.05 part by weight of t-dodecylmercaptan, to thereby obtain a feed ⁇ tock for polymerization.
• the thus obtained feed stock was introduced to a polymeri ⁇ zation reactor.
• a polymerization reaction was started at 105 °C with stirring, and the reaction was conducted for 3 hours. Then, the temperature was elevated to 130 °C, and the reaction was continued for 2 hours. Further, the temperature was elevated to 145 °C, and the reac ⁇ tion was continued for 1 hour.
• the resultant reaction mixture was introduced to a volatilization apparatus which was heated in vacuo, to thereby remove the unre- acted styrene and benzene to obtain a rubber-modified styrene polymer.
• the obtained polymer was introduced to an extruder having a plurality of dies attached thereto, and then, extruded into strands.
• the obtained strands were immediately cooled with water, and the cooled strands were cut into pellets .
• the obtained pellets of the rubber-modified styrene polymer were designated as HIPS-1.
• the butadiene content of HIPS-1 was calculated from the mass balance of the above- mentioned styrene-butadiene block copolymer and sty ⁇ rene, and found to be 9 wt% .
• HIPS-1 and a polystyrene resin were blended in a ratio of 100:30.
• the obtained blend was subjected to melt kneading by means of a 30 mmtf, single screw extruder.
• the resultant rubber-modified styrene poly ⁇ mer was designated a ⁇ HIPS-2.
• the butadiene content of HIPS-2 wa ⁇ calculated from the mass balance of the above-mentioned HIPS-1 and styrene polymer, and found to be 7 wt%.
• Foamable beads were prepared using a machine for extrusion and impregnation (hereinafter frequently referred to as "extrusion/impregnation machine” ) , which has a structure mentioned below.
• the extrusion/impregnation machine is equipped with a device for feeding a foaming agent under pres ⁇ sure.
• the foaming agent-feeding device communicates, through a conduit, to a melt-kneading site of the machine at which melt-kneading of the polymer is to be conducted.
• the extrusion/impregnation machine is also equipped at a front end portion thereof with a device for cooling an impregnated polymer to an appropriate temperature for extrusion and a number of dies (each having a diameter of 0.7 mm) for extruding an appropri ⁇ ately cooled, impregnated polymer.
• HIPS-1 was introduced to the extru ⁇ ion/impregna- tion machine at its melt-kneading site and melted therein. 0.13 mol of isopentane (per 100 g of HIPS-1) was fed as a foaming agent from the foaming agent-
• the resultant molten, isopentane-impregnated polymer wa ⁇ cooled to an appropriate temperature for extrusion by means of the cooling device and then, extruded in water at 60 °C through the above-mentioned ?n extrusion dies, and the extrudate is immediately cut in the water using a rotary cutter to obtain foamable beads having an average diameter of 1.1 mm.
• foamable beads were obtained in sub ⁇ tantially the ⁇ ame manner as 25 in the production of foamable beads from HIPS-1.
• the foamable beads of each of HIPS-1, -2 and -3 were expanded using a steam-foaming machine in a manner described below.
• the above-obtained expanded foamed beads were subjected to molding in a molding die provided in a conventional molding machine for producing a foamed polystyrene to effect fusion-unification with in. situ foaming of the beads.
• Ultimate foamed, molded products were obtained (bulk den ⁇ ity: 0.020 g/cm 3 ) having prede ⁇ termined shapes, which are intended for use as cushion-packings for packing up a 30 kg CRT monitor in a container.
• the thus obtained feed stock was introduced to a polymerization reactor.
• a polymerization reaction was started at 110 °C with stirring, and the reaction was conducted for 4 hours. Then, the temperature was elevated to 135 °C, and the reaction was continued for 2 hours. Further, the temperature was elevated to 150 °C, and the reac ⁇ tion was continued for 2 hours.
• the resultant reaction mixture was introduced to a volatilization apparatus, in which the mixture was heated in vacuo to thereby remove the unreacted styrene to obtain a rubber-modi ⁇ fied styrene polymer.
• the obtained polymer was desig ⁇ nated as HIPS-4.
• HIPS-4 had a butadiene content of 12.3 wt%, an [ ⁇ ] of 0.80, and an SWI of 9.5.
• the rubber particles disper ⁇ ed in the continuou ⁇ styrene polymer phase of HIPS-4 had a salami structure, and had an average particle diameter of 1.3 ⁇ m.
• substantially the same polymerization procedure as used for preparing HIPS-4 was repeated except that the concentration of the polybutadiene rubber in the solution thereof in styrene was changed to 5.5 wt%, and that reaction was conducted first at 110 °C for 4 hours, then at 135 °C for 2 hours, and then at 145 °C for 2 hour ⁇ , to thereby obtain a rubber-modified ⁇ tyrene polymer.
• the obtained rubber- modified ⁇ tyrene polymer was designated as HIPS-5.
• the properties of each of HIPS-4 and -5 are shown in Table 1.
• foamable o beads were prepared in substantially the same manner as in Example 1, except that the retention time of a molten mixture at 130 °C for the impregnation of the polymer with isopentane was changed to 25 minutes.
• the thus obtained foamable beads prepared respec- 5 tively from HIPS-4 and -5 were expanded with foaming and then, subjected to aging in substantially the same manner as in Example 1, to thereby obtain two types of expanded foamed beads each having an apparent density of 0.033 g/cm 3 .
• the average weight of each of the o obtained two types of foamed beads was 0.75 mg.
• the properties and appearance of the obtained two types of expanded foamed beads, respectively, obtained from HIPS-4 and -5 were found to be good as shown in Table 2.
• the above-obtained expanded foamed beads were subjected to molding in sub ⁇ tantially the ⁇ ame manner as in Example 1.
• Ultimate foamed, molded products were obtained (bulk density: 0.020 g/cm *3 ) having predeter ⁇ mined shapes, which are intended for use as cu ⁇ hion- packing ⁇ for packing up a 30 kg CRT monitor in a con ⁇ tainer.
• a polymerization was carried out in sub ⁇ tantially the ⁇ ame manner a ⁇ in Example 1.
• the obtained rubber-modified ⁇ tyrene poly ⁇ mer (I) had a butadiene content of 9 wt% and an average particle diameter of 0.2 ⁇ m, and the rubber particles dispersed therein had a core-shell structure.
• a polybutadiene rubber having a 1,4-cis content of 96 wt% was dis ⁇ olved in monomeric ⁇ tyrene so that the concentration of the polybutadiene rubber in the resultant solution became 9 wt%.
• a polymerization was carried out in substantially the same manner as in Example 2.
• the obtained rubber-modified styrene polymer (II) had a butadiene content of 12 wt% and an average particle diameter of 1.4 ⁇ , and the rubber particles disper ⁇ ed therein had a ⁇ alami structure.
• the above-obtained rubber-modified styrene poly ⁇ mers (I) and (II) were blended in a ratio of 9:1, and the obtained polymer blend was designated as HIPS-6. Further, a styrene-butadiene block copolymer having a butadiene content of 60 wt% was dissolved in monomeric styrene so that the concentration of the styrene-butadiene block copolymer in the resultant solution became 10.5 wt%. Using the resultant solu ⁇ tion, a polymerization was carried out in substantially the same manner as in Example 1.
• the obtained rubber- modified styrene polymer (III) had a butadiene content of 8 wt% and an average particle diameter of 0.3 ⁇ m, and the rubber particles dispersed therein had a core- shell structure.
• 1,4-cis content of 96 wt% was dissolved in monomeric styrene so that the concentration of the polybutadiene rubber in the re ⁇ ultant ⁇ olution became 6 wt%.
• a polymerization was carried out in sub ⁇ tantially the same manner as in Example 2.
• the obtained rubber-modified styrene polymer (IV) had a butadiene content of 8 wt% and an average particle diameter of 1.7 ⁇ m, and the rubber particle ⁇ dispersed therein had a salami ⁇ tructure.
• the obtained rubber-modified styrene polymer (IV) had a butadiene content of 8 wt% and an average particle diameter of 1.7 ⁇ m, and the rubber particle ⁇ dispersed therein had a salami ⁇ tructure.
• modified styrene polymers (III) and (IV) were blended in a ratio of 8:2, and the obtained polymer blend was designated as HIPS-7.
• HIPS-6 and -7 The properties and appearance of each of HIPS-6 and -7 are shown in Table 1.
• Example 1 Substantially the same procedure a ⁇ in Example 1 (2) wa ⁇ repeated to obtain foamable bead ⁇ from HIPS-1 mentioned in Example 1 (1) .
• the obtained foamable beads were expanded in the same manner as in Example 1
• condition 1 in which the temperature of 102 °C was maintained for 30 seconds
• condition 2 in which the temperature of 102 °C was maintained for 20 seconds
• condition 3 in which the temperature of 102 °C was maintained for 15 seconds.
• Expanded, foamed beads obtained by using condition 1 above had an apparent density of 0.018 g/cm
• expanded, foamed beads obtained by using condition 2 above had an apparent density of 0.023 g/cm 3
• expanded foamed beads obtained by using condition 3 above had an appar ⁇ ent density of 0.040 g/cm 3 .
• the foamed beads having an apparent density of 0.018 g/cm had an apparent density of 0.018 g/cm
• expanded, foamed beads obtained by using condition 2 above had an apparent density of 0.023 g/cm 3
• expanded foamed beads obtained by using condition 3 above had an appar ⁇ ent density of 0.040 g/cm 3 .
• Example 4 Substantially the same procedure as in Example 4 was repeated except that HIPS-4 was used in place of HIPS-1, thereby obtaining three different types of expanded foamed beads and foamed, molded products corresponding thereto. With respect to each of the three types of foamed beads and each of the correspond- ing molded products, various properties are shown in Table 3. Comparative Example 1
• HIPS-8 had an [ ⁇ ] value of 0.52 and an SWI of 10.5.
• Expanded foamed beads and a foamed, molded product corresponding thereto were prepared from the above HIPS-8 in substantially the same manner as in Example 1. The properties of the above-mentioned foamed beads and foamed, molded products are shown in Table 4.
• the ratio b/a was 8
• the ratio a/c was 0.06
• the average weight was 0.65 mg.
• the above-obtained foamed beads were poor in the retention of a foaming agent gas. Further, the foamed, molded product obtained therefrom was unsati ⁇ factory in cracking re ⁇ i ⁇ tance and appearance. Comparative Example 2
• HIPS-10 HIPS-10.
• HIPS-10 had an [ ⁇ ] value of 0.85 and an SWI of 4.5.
• Expanded foamed beads and a foamed, molded product corresponding thereto were prepared from the above HIPS-10 in substantially the same manner as in Example 1.
• the properties of the above-mentioned foamed beads and foamed, molded product are shown in Table 4. With respect to the obtained foamed beads, the ratio b/a was 7, the ratio a/c was 0.06, and the average weight was 0.70 mg. As is apparent from Table 4, the foamed beads were poor in the retention of a foaming agent gas. Further, the foamed, molded product obtained from the above-obtained foamed beads was unsatisfactory in cracking resistance and appearance.
• the ratio b/a wa ⁇ 43, the ratio a/c wa ⁇ 0.02, and the average weight was 0.68 mg.
• the foamed beads were poor in the retention of a foaming agent gas.
• the foamed, molded product obtained from the above formed beads was unsati ⁇ factory in cracking resistance and appearance. Comparative Example 5 o Using HIPs-1, -4, and -6 individually, substan ⁇ tially the same procedure as in Example 1 (2) and (3) was repeated except that, in Example 1 (2), the reten ⁇ tion time in the melt-kneading site at 130 °C was changed to 5 minutes, to thereby obtain expanded foamed 5 beads and a foamed, molded product.
• Expanded foamed beads and a foamed, molded product were prepared from the above HIPS-12 in sub ⁇ tantially the 5 ⁇ ame manner as in Example 1.
• the properties of the obtained expanded foamed beads and foamed, molded product are shown in Table 5.
• the ratio b/a was 35
• the ratio a/c was 0.009
• the average weight was 0.70 mg.
• the molded product obtained from the above expanded foamed beads was unsati ⁇ factory in cracking re ⁇ istance. Comparative Example 7
• Expanded foamed beads and a foamed, molded product were prepared from the above HIPS-13 in substantially the same manner as in Example 1.
• the properties of the above-obtained ex ⁇ panded foamed beads and foamed, molded product are shown in Table 5.
• the ratio b/a was 18, the ratio a/c was 0.21, and the average weight was 0.75 mg.
• the above-obtained foamed beads were poor in the retention of a foaming agent gas and the ratio of closed cells. Further, the foamed, molded product obtained from the above foamed beads was unsati ⁇ factory in cracking resistance and appearance. Comparative Example 8
• Example 1 Substantially the same procedure as used for o preparing HIPS-2 in Example 1 (1) was repeated except that the ratio of HIPS-1 to a polystyrene resin was changed to 100:200.
• the resultant rubber-modified styrene polymer was designated as HIPS-14.
• the butadi ⁇ ene content of HIPS-14 was calculated from the mass 5 balance of the HIPS-1 and styrene polymer, and found to be 3 wt%.
• Expanded foamed beads and a molded product were prepared from the above HIPS-14 in substantially the same manner as in Example 1. The properties of the obtained expanded foamed beads and foamed, molded o product are shown in Table 5.
• the ratio b/a wa ⁇ 16, the ratio a/c wa ⁇ 0.04, and the average weight was 0.66 mg.
• the rubber particles are not dispersed in a lamellar 5 configuration in the cross-section of the cell wall.
• the molded product obtained therefrom was unsatisfacto ⁇ ry in cracking resistance. Comparative Example 9
• Example ' 4 Substantially the same procedure as in Example ' 4 was repeated except that HIPS-10 was used in place of HIPS-1, thereby obtaining three types of foamed beads having apparent densities of 0.018 g/cm 3 , 0.023 g/cm 3 and 0.040 g/cm 3 , respectively. These three types of foamed beads were subjected to molding in substantially the same manner as in Example 4, thereby obtaining three types of molded products having bulk densities of 0.11 g/cm 3 , 0.14 g/cm 3 and 0.24 g/cm 3 , respectively.
• the ratios b/a were, respectively, 9, 8 and 7; and the ratios a/c were, respectively, 0.21, 0.15 and
• the expanded foamed bead of a rubber-modified stylene polymer of the present invention can be used for producing a molded product which is excellent in cracking resistance.
• the molded product can be advan ⁇ tageously used a ⁇ a cushioning material in packaging an article having a relatively large weight which is likely to be repeatedly subjected to a jolt or impact during transportation. Further, due to the excellent resi ⁇ tance to cracking, the amount of the cu ⁇ hioning material to be used can be reduced, so that the volume of the packaged article is reduced, thus improving the efficiency of the transportation.
• a molded product produced from the expanded foamed bead of the present invention is excellent also in flexibility, so that it can also be advantageously used a ⁇ , for exam- pie, a heat insulating material for use in, for exam ⁇ ple, houses and various types of bath ⁇ .
• the expanded foamed bead of a rubber-modified styrene polymer of the present invention is excellent in the retention of a foaming ga ⁇ and, therefore, exhibits a high expanding capability, so that a molded product produced therefrom is excellent in appearance.
• the expanded foamed bead of the present invention and a molded product thereof are advantageous in that they can readily be produced at a relatively low cost by the use of customary equipment.
• a molded product produced from the foamed bead of the present invention is melted together with a molded product produced from conventional foamed bead ⁇ , the miscibility therebetween is good, and the resultant polymer blend can be pelletized for further use.
• the expanded foamed bead of a rubber-modified styrene polymer of the present inven ⁇ tion is extremely useful in fields relating to foamed, molded products of expanded foamed beads.
• Rubber- Intrinsic Swelling Structure Average Butadiene modified viscosity of index of of rubber diameter content of styrene continuous gel moiety particles of rubber rubber-modified polymer styrene poly ⁇ particles styrene polymer No. mer phase [ ⁇ ] ( ⁇ un) (wtZ)
• HIPS-1 0.68 10 core-shell 0.2 9.0
• HIPS-3 0.69 11.5 core-shell 0.2 10.5
• HIPS-2 was actually prepared by melt-kneading HIPS-1 and a polystyrene resin blended in a ratio of 100:30 in Example 1.
• HIPS-14 was actually prepared by melt-kneading HIPS-1 and a polystyrene resin blended in a ratio of 100:200 in Comparative Example 8.

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